The increased concern for health standards in consumer product packaging requires that containers often be pasteurized before they are filled. This makes the application of labels to some containers especially difficult because the labels typically are applied by wrap around labeling to a container before the container is filled. After the label is applied, the empty container is pasteurized. The high heat of pasteurization could cause a hot melt adhesive to weaken and cause heat distortion of the label with the result that the adhesive bond would break. This problem could occur with hot beverage filling, where the containers are filled under heat with a beverage.
One type of hot melt adhesive, comprising a polyurethane based polymer, has been found advantageous for use on an adhesive based joint for a label applied by wrap around labeling and which is subject to high heat such as a pasteurization process. An example of such adhesives are those adhesives disclosed in U.S. Pat. Nos. 4,608,418 and 4,870,142 to Czerwinski et al., dated Aug. 26, 1986 and Sep. 26, 1989 respectively. The adhesives disclosed in these patents are advantageous because they can withstand high heat such as from a pasteurization process or hot beverage filling, while maintaining the label bond even when the label is subject to heat induced forces. Thus, the heat induced forces will not tear the adhesive bond.
Such adhesives, however, are extremely tacky, and the label is prone to adhere to a glue roller when a glue roller engages the label for adhesive transfer. The label could be pulled from the label drum. Additionally, such hot melt adhesives have a viscosity which tends to increase if the adhesive is not kept in constant motion or is allowed to set. Thus, slight delays of a few seconds in which the adhesive is not agitated or sheared during transfer from glue bars onto glue rollers, or other contact points, could cause an increase in adhesive viscosity which would make adhesive transfer difficult or impossible.